1. Field of the Invention
The present invention relates generally to a switching control circuit, and more specifically to a switching control circuit used for a switching power converter.
2. Description of Related Art
A power converter is used to convert an unregulated power source to a regulated voltage or current source. The power converter normally includes a transformer or a magnetic device having a primary winding and a secondary winding to provide isolation. A switching device connected in the primary winding to control energy transfer from the primary winding to the secondary winding. The power converter operates at a high frequency for allowing size and weight reduction.
However, the switching operation of the switching device will generate switching losses and electric-magnetic-interference (EMI). FIG. 1 shows a fly-back power converter and the waveforms are shown in FIG. 2. The switching device Q1 is applied to switch a transformer T1 and control the power delivered from the primary winding NP to the secondary winding NS of the transformer T1. Energy is stored in the transformer T1 when the switching device Q1 is turned on. As the switching device Q1 is switched off, the energy of the transformer T1 is discharged to the output of the fly-back power converter through a rectifier DS. In the meantime, a reflected voltage signal VR is generated in the primary winding NP of the transformer T1 in accordance with the output voltage VO and the turn-ratio of the transformer T1. Therefore, the voltage VD across the switching device Q1 is equal to the input voltage VIN plus the reflected voltage signal VR once the switching device Q1 is turned off. The energy from the voltage VD is stored in the imaginary parasitic capacitor CQ, corresponding to the parasitic capacitance of the switching device Q1. After a discharge period TDS, the energy of the transformer T1 is fully discharged, the energy stored in the parasitic capacitor CQ flows back to the input voltage VIN through the primary winding NP of the transformer T1. The parasitic capacitor CQ and the primary winding inductor (not shown) of the transformer T1 develop a resonant tank, wherein its resonant frequency fR can be shown as equation (1),
                              f          R                =                  1                      2            ⁢            π            ⁢                                                            L                  p                                ×                                  C                  j                                                                                        (        1        )            
Wherein, Cj is the capacitance of the parasitic capacitor CQ; LP is the inductance of the primary winding inductor of the transformer T1.
During the resonant period, the energy of the parasitic capacitor CQ is delivered to the primary inductor of the transformer T1 back and forth. There is a delay time Tq corresponding to the time the parasitic capacitor CQ takes to discharge until the voltage VD reaches a minimum value. The delay time Tq is the period of the quasi-resonance and it can be expressed as equation (2).
                              T          q                =                  1                      4            ×                          f              R                                                          (        2        )            
Sum up, if the switching device Q1 is turned on during the valley voltage across the switching device Q1, which can achieve the soft switching so as to minimize the switching loss and EMI of the power converter.